1. Field of the Invention
The present invention relates to heating and temperature cycling. More specifically, the present invention is drawn to an apparatus for heating samples and a temperature cycling apparatus incorporating the device for heating the samples.
2. Description of the Related Art
During the handling of various chemical samples, it is often necessary to heat the samples to effect a desired reaction or result. In addition, it is often desirable to subject the chemical samples to temperature cycling through a range of hot and cold temperatures.
Devices for heating chemical samples currently employ a solid block of aluminum having wells drilled therein to receive vials which hold chemical samples. The vials holding the samples are ordinarily made from a plastic such as polypropelene. The wells drilled in the aluminum block have close tolerances to the outer dimensions of the plastic sample vials so as to provide significant contact between the outer surfaces of the vials and the aluminum block. Furthermore, the tight tolerances reduce the losses in heat during transfer from the block to the vials and chemical samples.
The solid aluminum block is heated which, in turn, heats the chemical samples contained in the tubes that are positioned in the wells of the solid block. The aluminum block is heated using an electric heater having uniform heat dissipation. Due to the mass of the solid block, the electric heater requires a power rating of approximately 1000 Watts. The solid block arrangement provides for substantially uniform heating of the samples held therein.
When electric heating is employed, the heating element which is placed into contact with the solid aluminum block has decreased heat transfer characteristics near the edges of the electric heating element. As a result, the heat transferred to the solid block is not uniform and heating efficiency of the solid block is reduced. Furthermore, the current required to operate the electric heater is substantial, thus increasing operating costs.
In order to prevent evaporation of the samples contained in the plastic tubes, heated covers are placed over the plastic tubes. These covers are heated to a temperature which exceeds the temperature to which the samples contained in the tubes are heated. As a result, evaporation of the sample is prevented. However, since the tubes are normally filed to approximately 1/3 of capacity, separation of the sample contained within the tube often occurs. More specifically, the sample travels up the walls of the plastic tube until it is repulsed by a higher temperature level. The greater the distance between the sample and the higher temperature gradient, the further the sample travels in the tube. As a result, therefore, the sample vials must be subjected to a centrifuge to gather the samples.
When the solid aluminum block is incorporated into a temperature cycling arrangement, the heated block containing the sample is normally immersed in a cold bath to reduce the temperature of the block and the chemical samples held therein.
The drawbacks associated with the solid block construction include high manufacturing cost, high weight, and complicated cooling methods to achieve acceptable cooling times. The high weight of the solid block also proves inconvenient if the solid block is to be placed in a cold bath to effect cooling.
Accordingly, it is an object of the present invention to provide a light weight sample holding device which provides good heat transfer characteristics to the samples held therein. It is a further object of the present invention to minimize separation of chemical samples held within tubes associated with the sample holding device. An additional objective of the present invention is to provide a light weight and small size temperature chamber to effect temperature cycling of chemical samples. It is also an object of the present invention to provide convenient and effective cooling of samples when subjected to temperature cycling. An added object is to provide an improved heating element construction which increases heat transfer characteristics near the edges of the heating element.